This invention relates to button forming and more particularly to injection molding a button head on a preformed metallic shank.
Providing buttons having a preformed shank consisting of one or more protruding legs to be inserted into a second button part, and a head portion covered with a plastic material are well known in the art. For example, such a button construction is shown in U.S. Pat. No. 4,580,320 which describes a method for forming such a plastic covered button by first preforming the button shank and also preforming the plastic head covering and then subsequently snapping the head covering onto the head. Such an arrangement has many disadvantages. First, precise tolerances must be maintained between the size of the metallic head and the opening preformed in the plastic covering to assure that the plastic head covering will in fact snap onto the head and be retained thereon. If the metallic head is too large or the opening is too small, the cap will not easily be inserted onto the metallic head. If the metallic head is too small or the cap opening too large, the cap will not be retained on the head. Further, forming both the metallic portion and plastic portion as separate pieces and later joining them results in assembly difficulties in requiring that the small plastic caps be oriented and held precisely in order to cause them to be snapped onto the metallic heads.
Additionally, the metallic head must be symmetric (round) in order to be easily assembled with the plastic cap. It would be very difficult or impossible to assemble the shank head as it exists to a plastic cap. Surrounding the shank head with plastic as taught by the present invention is an excellent way to hold the shank head in the plastic cap.
Finally, the shank geometry makes it impossible to "insert mold" this item according to known molding methods. The only way to effectively seal the shank leg in the molding operation is to pierce a metal strip with the shank leg, as now taught by the present invention, thus effecting a seal.